Epoxy resins have excellent moisture, solvent, chemical and heat resistance properties, good adhesion, and superior mechanical and electrical properties, which make epoxy resins suitable for use in constructing and packaging electronic products. In many cases, flame retardant materials are included in the epoxy resin compositions for use in electronic applications and/or electronic components that require high flame retardancy.
In one approach, the flammability of the epoxy resin can be reduced by physically blending a flame-retardant additive with the epoxy resin. Some examples of such additive-type flame retardants include antimony trioxide, aluminum trihydroxide, elemental phosphorous, and inorganic phosphorous compounds. Unfortunately, such additive-type flame retardants may be toxic. Additionally, such additive-type flame retardants may be difficult to integrate with the epoxy compositions, thereby, necessitating a high initial loading of the additive-type flame retardants that adversely influences the electrical or mechanical properties of the epoxy resin.
One commonly used flame-retardant in epoxy compositions for printed wiring boards (PWB) in electronic equipment is a halogenated aromatic flame-retardant, such as a brominated aromatic flame-retardant. The brominated aromatic flame-retardant, for example, tetrabromobisphenol A (TBBPA) based compounds, chemically bond with the polymer chain. Unfortunately, such halogenated aromatic flame-retardant may emit corrosive halides and toxic compounds during a fire. Additionally, there has been an increased global interest in environmental protection leading to a higher demand for halogen-free flame-retardants (HFFR) in the epoxy formulations for PWB.
Alternatively, phosphorous or nitrogen containing epoxies and/or epoxy curatives as flame retardants have been considered for improving flame retardancy. Unfortunately, relatively large quantities of phosphorous-based compounds are needed to provide for sufficient flame-retardancy, which quantities have been observed to greatly reduce heat and moisture resistance of the epoxy resins.
Nitrogen-based flame retardants are considered advantageous as they are observed to have a low toxicity, are physically stable, and in case of fire, have an absence of toxic and corrosive emissions with a low evolution of smoke. More recently, triazine-phenol-aldehyde (TPA) condensates have been described as flame retardant additives for epoxy resins. TPA condensates with high nitrogen content are also effective curing agents for epoxy resins leading to high efficiency in flame retardancy without compromising the mechanical and physical properties of the polymer.
However, one difficulty with TPA condensates is that the current condensate compositions exhibit higher viscosity and lower nitrogen content than desired. For example, existing processes typically produce an atomic nitrogen content of only about 1 to 10 wt. %. TPA condensates have also been observed to exhibit instability at higher temperatures that limit the large scale manufacturing of such condensates.
Conventional processes for forming TPA condensates have been found to be disadvantageous as the resulting condensates exhibited an undesirable increase in viscosity and also exhibited a decreased solubility in commons solvents, such as methyl ethyl ketone (MEK) and acetone, typically used in epoxy formulations. The solubility of the TPA condensates in such solvents is important since residues or insolubles in the epoxy resins may result in less than desirable coating of substrates and degrade the quality of the laminates made from the epoxy resins.
Other conventional TPA condensate formation processes prepare compounds with large amounts of methanol as a reactant, which present special challenges on commercial scale productions including handling, waste, and expense.
Therefore, there is a need for forming flame-retardant condensates, with improved viscosity and improved solubility that are effective curing agents, provide fire-retardant properties to epoxy compositions and which may be manufactured on a commercial scale.